Volumetric flow rates at which gases travel through closed pipes are at times measured by placing a flowmeter directly in the pipe flow path. When relatively low levels of flow, e.g., two to fifty cubic feet per minute (CFM), are monitored in relatively large pipes, e.g., pipes six or more inches in diameter, the low level flow may be bypassed through a small pipe in order to increase the velocity of fluid flow and thereby improve the accuracy of measurement. For example, air inleakage in steam turbines is actively exhausted in order to minimize corrosion of turbine components and to reduce vibration in low pressure turbine blading. Efforts are made to keep air inleakage rates below ten CFM during normal operating conditions, but rates may range up to 2800 CFM during turbine start up. High compression ratio pumps designed to exhaust this wide range of flow must operate under low back pressure. Otherwise, excessive exhaust back pressures may damage the pump seals. Consequently, the vent pipes which exhaust this air are at least six inches in diameter and may be larger for long pipe lengths in order to minimize rises in back pressure when the pumps displace large volumes of air.
Velocity based flowmeters which have been used for measuring volumetric exhaust rates in these vent pipes have required a minimum flow velocity of approximately 50 feet per minute in order to maintain an acceptable level of accuracy. However, the velocity of a one CFM flow through a six inch pipe is on the order of only five feet per minute. Therefore it has been necessary to bypass turbine exhaust air through a flow monitor having a markedly smaller inside diameter than the vent pipe in order to bring the exhaust air velocity into an acceptable range for measuring volumetric flow rates. Similar constraints exist for positive displacement flowmeter but for different reasons. This latter type flowmeter reads low rates (2-50 CFM) with good accuracy but must be bypassed at higher flow rates to reduce flow through the meter. When a flowmeter is connected in parallel with the vent pipe, a bypass valve used to divert exhaust flow to the flowmeter must completely seal off the vent pipe in order to effect continuous monitoring. When the exhaust flow rate increases, e.g., due to a sudden inleakage of air to the turbine system, the bypass valve must be quickly opened in order to avoid excessive back pressure which would result from moving the increased volume of gas through the relatively small diameter flowmeter.
Applicant has developed one form of bypass valve which accomplishes this quick opening using a bladder valve as a control valve. The bladder valve is essentially an oblong balloon or bladder fixed in position within a flow pipe such that it can be expanded or contracted by control of gas pressure in the bladder. In this valve, operation was based on differential pressure measurements, i.e., pressure in the vent pipe is used to control bladder valve operation. It has been found that this form of control may result in oscillations of the control valve and may also provide a misleading low gas flow reading when the control valve is open.